Field of the Invention:
The invention relates to a metallic combustion air heater, herein referred to as NQAPH (No Quench Air Pre Heater), located immediately after the reaction zone of a carbon black reactor and subject to high temperature of the reactor effluent, around 1300 C (2372 F). The invention, (NQAPH), eliminates the water quench, serves as the apparatus to stop the pyrolysis and cools the effluent sufficiently to protect downstream equipment including the carbon black collector; the invention has higher heat transfer rates and results in a smaller combustion air heater compared to the prior art (APH). Quench water to stop the pyrolysis is eliminated, saving associated energy loss. (162 kg/h of quench water and 123 kW of energy per 1000 nm3/h of combustion air for a typical 950 C combustion air heater)
Description of the Prior Art
In the prior art, carbon black by furnace process is made by creating a high temperature flame (around 1925 C) in an adiabatic reactor, firing fuel into a stream of hot combustion air from a combustion air preheater; injecting Carbonaceous feedstock into the flame and make carbon black and other gases by pyrolysis. The Carbon black containing gases compose of CO, H2, CO2, CH4, C2H2, N2, H2O and traces of H2S, COS and is known as the reactor effluent. When the right quality of Carbon black is made, the pyrolysis should be stopped by quickly cooling the effluent immediately after the reaction zone. In the prior art mostly water has been used to quench the reactor effluent to arrest pyrolysis and to cool the effluent further to protect the downstream equipment, typically combustion air heater, (APH). Such water has to be treated to reduce the impurities content for carbon black product quality as well as trouble free operation of downstream equipment (due to the deposition of the impurities on the heating surfaces). This water, an increasingly important commodity, is converted to superheated vapor in this cooling process and the water and energy are irrecoverably lost to the atmosphere.
Besides, the with poor atomization of the water, tiny droplets of water may entrain the carbon black and deposit them on the inside of the tubes of the combustion air preheater and cause fouling of the tubes and reduce the heat transfer effectiveness of the air heater and other downstream heat exchangers.
The effluent, inadequately cooled due to the fouling, will increase in temperature entering the bag collector, and will require more water quench to protect the carbon black collector.
To eliminate the water quench in carbon black manufacture by furnace process, borrowing from the Ethylene production process, quench boilers have been used to stop the pyrolysis and cool the effluent to safe operating temperature for the combustion air preheater. These quench boilers use high pressure water as the cooling medium and generate high pressure steam which may be used in the process or generate electrical power.
In U.S. Pat. No. 465,270, Mills et al use water quench only to stop the pyrolysis and further cooling till the next heat exchanger is done by pumping required amount of cooled effluent from upstream of the carbon black collector.
Per U.S. Pat. No. 6,086,841, Lee has proposed to eliminate the water quench and stop the pyrolysis and cool the reactor effluent by means of a heat exchanger lined with silicon carbide. The effluent exiting the lined heat exchanger enters the combustion air heater, followed by secondary cooler and carbon black collector. The heat from the lined heat exchanger is lost to the atmosphere or transferred to another fluid stream.
The prior art combustion air heater, (APH), typically a shell and tube counter current unit, has the hot reactor effluent from the reaction zone, cooled adequately with water quench to stop the pyrolysis and reduce the effluent temperature safe for the combustion air heater (APH), flowing inside multiple vertical tubes at high velocities, typically around 80 m/s. Such high effluent velocities and vertical orientation of tubes are needed to keep the carbon black in suspension and minimize the fouling on the tube walls which will hinder good heat transfer. The tubes are housed inside an enclosure bounded by a metallic shell and tube sheets at the hot and cold ends of the APH.
The combustion air, facilitated by baffles inside the shell, flows over the tubes in multiple passes; the air flows partly in cross flow, transvers to the tubes and partly in longitudinal flow parallel to the tubes.
Higher combustion air temperatures result in lower fuel consumption to the combustion section of the reactor, increased O2 availability in the flame, higher yield and increased production rate of carbon black. To obtain these benefits of economy in carbon black manufacture, the combustion air temperature for the furnace process gradually increased from 450 C in the 1970′s to 650 C & 800 C through the 90′s to the current 950 C. Improved higher temperature materials may, in the future, push the combustion air temperature to 1000 C and beyond. These increases have been achieved in the prior art APH, by higher effluent inlet temperatures to the combustion air preheater by reduced quench.
Present Invention:
In the present invention, the metallic combustion air heater, NQAPH, is located immediately after the reaction zone, eliminating the water quench. The effluent containing carbon black, at high temperature (typically around 1300 C +) enters the inside of the vertical tubes at high velocity (typically 80 m/s). Combustion air flows over the tubes in multiple passes, facilitated by segmented, disc and donut type baffles.
In the present invention, the water quench in the prior art to stop pyrolysis and control the hot air temperature of the combustion air heater, is eliminated. The NQAPH replaces the lined heat exchanger per Lee which dissipates the heat to the atmosphere or other fluid, before flowing into the combustion air heater. The present invention operating with lower effluent flow due to the absence of the water quench and at high temperature results in a smaller combustion air heater than in the prior art for the same combustion air temperature (950 C).
In the present invention, the absence of water quench reduces the volume of effluent from the reaction zone by about 10.5%. This reduced effluent flow at 1300 C and the 950 C combustion air temperature results, by heat balance, in effluent exiting the combustion air heater at 865 C.
An all co current air heater (with the effluent and combustion air flowing co currently) is not feasible as the air temperature 950 C. is higher than effluent exit temperature of 865 C.
An all counter current air heater is feasible, with the combustion air at 950 C exiting the air heater at the point where the hot effluent at 1300 C is entering. However, this will subject the metallic tube to a maximum temperature, about 1135 C, exceeding the usable limit of commercial metals. Additionally, the top and bottom plates of the double plate bottom tube sheet will be hotter than the top tube sheet, resulting in the tube holes of the top and bottom tube sheets not being aligned and failure of the seals between the tube and the top tube sheet.
To obviate these problems, the present invention is a combination co current counter current combustion air preheater, wherein the hottest tube is not at the hottest entry of the effluent, but at a location between the top and bottom tube sheets, where the effluent has cooled sufficiently. Such splitting of the cooling medium into two streams, with one stream co current and the other counter current, to keep the hottest metal temperature away from the hottest entry temperature of the heating medium has been used extensively by those skilled in the art, as shown by Cox (U.S. Pat. No. 2,869,830), by Schack ((U.S. Pat. No. 2,917,285) and by Cameron (U.S. Pat. No. 5,477,846).
In the present invention, the total combustion air is split into three streams in the heat exchanger. As shown in FIG. 1, the stream 1, approximately 50% of the total air flow will enter the shell of the air heater (NQAPH) at the hot end, near the double plate bottom tube sheet and flow over the tubes in multiple passes co-current to the reactor effluent. The second stream, approximately 35 to 40% of the total air flow will enter the shell of the air heater (NQAPH) at the colder end, near the top tube sheet, and flow over the tubes in multiple passes counter-current to the reactor effluent. The third stream, approximately 15% to 10% of the total air flow will flow through the double plate bottom tube sheet (to cool the tube sheet plates) and flow out of the top plate of the double plate bottom tube sheet, through the inside of one or more cooling air return pipes and join the second stream of air at the colder end of the air heater (NQAPH). This third stream picks up heat from the effluent carrying tubes, connected to the double plate bottom tube sheet as well as the heat radiated by the adjacent hot effluent carrying tubes to the cooling air return pipe(s).
When this warm stream 3, mixes with stream 2 of air, the combined stream is warmer than the entering second stream and helps reduce the fouling tendency, by keeping the effluent carrying tubes near the top tube sheet, warmer. The co-current and counter current air streams pick up heat from the hot tubes inside which the hot reactor effluent is flowing and join together about the middle and exit the air heater (NQAPH). At this location, the effluent has been cooled to a temperature of about 1085 C and the tube materials are at about 1025 C, well within the usable limits of metals. The hot process air flows through the external hot air piping to the reactor.
In the present invention, the heat transfer flux increases by about 34%, the length of the tubes is reduced by about 25%, making the process air heater 25% shorter and making the usually vertically oriented air heater, structurally more stable.
In the present invention, unlike the prior art (APH) where the hot exiting air is in contact with the top plate of the double plate bottom tube sheet, cold incoming air of streams 1 and 3 are in direct contact with the double plate bottom tube sheet, keeping the tube sheet plates cooler and therefore, stronger.
In the present invention, the tube to bottom tube sheet weld, (FIG. 2), being in contact with the cold incoming air of stream 3, is, therefore, stronger and minimizes the potential for cracking and failure. The stream 3 flow is controllable by a control valve, based on the measured tube sheet plate temperature and the measured temperature of stream 3 exiting the double bottom tube sheet.
In the present invention, the shell, due to full length internal and external insulation, operates at lower temperature than in the prior art (APH) where only part of the shell is internally and externally insulated. (FIG. 6 shows the shell and tube temperatures for the NQAPH and prior art APH)
In the present invention, due to the shorter length of the unit, as in (006) above, and lower temperature, as in (009) above, the shell thermally expands less than the shell in the prior art (APH). The differential expansion between the tube and shell is also lower than in the prior art (APH). (FIG. 7 shows the thermal expansions of the tube, shell and the differential between them for the NQAPH and prior art APH)
In one configuration, the shell of this airheater (NQAPH) is fully externally and internally insulated. The internal insulation is retained in place by means of metal pins and metallic liner. The process air flows over this metallic liner.
In another configuration, the shell is only externally insulated and is provided with an internal metallic liner. The second stream of air will enter the airheater at the hot end and flows to the colder end through the annulus between the shell and the inner metallic liner. Typically, turbulators are provided in the annulus to improve heat transfer and structural stability of the inner liner which is subject to the pressure of the combustion air on the outside of the inner liner.
In one configuration, the internal baffles to create multiple passes for the airheater (NQAPH) are of the segmented type.
In another configuration, the internal baffles to create multiple passes for the airheater (NQAPH) arc of the disc and donut type or any other method to create multiple passes of the process air.
In one configuration, the tubes are connected to the top tube sheet at the colder end of the airheater (NQAPH) by means of packing seals (FIG. 3), with the tubes free to slide inside the seals.
In another configuration, the tubes are connected to the top tube sheet at the colder end of the airheater (NQAPH) by means of metallic bellow type seals (FIGS. 4 and 5), welded to the sleeves in the top tube sheet or by any other method of sealing the process air from mixing with the hot reactor effluent, allowing for the hot tubes to thermally expand freely.
The process air for the CB production may be ambient air, Oxygen enriched air or 100% Oxygen.